Cellular ground networks are generally formed of a plurality of base transceiver stations (BTSs) which wirelessly contact mobile units serviced by the network. The BTSs are connected in a tree topology to a regional base station controller (BSC) which controls the routing of calls in the network. Usually, a single BSC is connected to tens or hundreds of BTSs. The BSC is generally connected to a mobile switching center (MSC) which in turn is connected to other BSCs and to one or more public telephone networks. When a call is established with a mobile unit, a channel is generally allocated between the mobile unit and the MSC through a BSC and one or more BTSs, to pass the signals of the call to and from the mobile unit.
Cellular networks can be used to transmit data packets, in addition to phone calls. A protocol for such data transmission is referred to as a general packet radio service (GPRS) and is described, for example, in an emerging ETSI draft standard for GSM, GSM 03.60 version 7.4.0 Release 1998, the disclosure of which is incorporated herein by reference. In accordance with the GPRS, the BSCs are connected to GPRS support nodes (GSNs) which pass the data packets to their destinations on IP networks. For example, the GSNs may serve as gateways to the Internet. In many cases the GSNs are located adjacent to or within respective MSCs.
Generally, mobile data connections are formed between mobile units requesting to transmit and/or receive data and the GSN. The connections pass partially through over a wireless link between a BTS and the mobile unit and partially on a link connecting the BTS to the GSN. The GSN generally includes an array of dedicated ASIC processors which each handles a single mobile data connection. The number of ASIC processors positioned in a GSN is limited by various physical constrains, such as the space required for the processors and their cost. For each active data connection, the GSN manages a serving GPRS support node (SGSN) which controls the connection and a gateway GPRS support node (GGSN) which serves as an interface between the mobile network and a packet based network, i.e., an IP network. The GGSN generally compresses the signals transmitted to the mobile unit, in order to conserve wireless bandwidth.
Some service providers use for the channel connecting the GSN to an IP network a link with a bandwidth lower than the total bandwidth of the connections from the GSN to the mobile units. This is possible because the connections between the GSN and the mobile units are not always fully utilized. The size of the link is chosen so that statistically, the chances of the bandwidth actually required by the mobile units is nearly never greater than the links bandwidth.